Home made mortar.
Take one banger .... the most powerful available to a schoolboy. Take one empty baked bean or golden syrup tin. Drill hole in tin near the open end, insert banger [fuse outside], tamp with clay or putty.
Make small earth mound ridge at right angles to target. Place bomb angled towards target.
Place additional bombs at varying angles to give a spread.
Light fuses.
Unwise to await fall of shot if the target greenhouse was hit.